i965/cfg: Track prev_block and prev_inst explicitly in the whole function
i965/cfg: Track prev_block and prev_inst explicitly in the whole function

This provides a trivial simplification now, and it makes some future
changes more straight forward.

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Francisco Jerez <5906ff32bdd9fd8db196f6ba5ad3afd6f9257ea5@riseup.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler